We are grateful to Mike Price and others who kindly pointed out the unfortunate error in equation (11) of our paper (Younger & Elliot 1995). Readers wishing to obtain K estimates in m/day should use the following revised version: which assumes laminar flow of water at 10°C and three mutually perpendicular sets of fissures with constant aperture (win m) and frequency (N in m-1).

Price has examined the impact of the error in equation (11) and its revision for our discussion and conclusions; we of course concede that our original equation (11) yields K values which are too low. Furthermore, we agree that the revised theoretical effective hydraulic conductivity of 98 m/day derived by Price from our Rn-derived estimate of fissure aperture (w = 0.45 mm, with N = 10 m-1) is too high (by a factor of about 100) for most of the confined Chalk. Price goes on to argue that this discrepancy shows that the cubic law usedby Snow (1968) (as given in equation 11) is fundamentally invalid for the natural fissures. However, we recognize another possibility: that the estimate of Chalk uranium content, [U]r, is too high. Both possibilities deserves scrunity.

We do not agree with Price that Sharp & Maini's (1972) work indicated that the reduction in hydraulic conductivity due to the morphology of real fractures as compared with smooth surface roughness is likely to be so great as to reduce the cubic exponent to 2 or 1.2 although the literature is somewhat equivocal. Tsang (1984)

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